The invention relates to a device for shed forming whereby the position of the heddles for the warp threads in a weaving machine are individually controlled according to the open-shed principle. In such a shed-forming device the heddles for the warp threads can occupy two positions: Bottom, i.e. below the weft insertion level, and Top, i.e. above the weft insertion level. It is called an open shed when each position can be reached or be maintained on every pick.
From the British patent publication GB 2 047 755 a shed-forming device for a weaving machine is known whereby the arcades are suspended from a pulley element. Around the wheel of this pulley element a cord is passed of which each extremity is connected to a leaf-spring-shaped hook. Each leaf-spring-shaped hook is provided on the bottom part with a nose with which it can rest on a corresponding lifting knife. The knives are brought two by two in opposite phase in an upward and downward movement. The leaf spring hooks are provided on top with a hook with which in a top position they can hook onto a fixed knife through the influence of an electromagnet which is placed between two leaf springs that work together. The nose of the bottom part of the leaf-spring-shaped hooks however always remains in the path of the ascending and descending lifting knives that work together. An unselected hook always remains on its corresponding lifting knife. The "bottom" positions for the leaf spring hooks are therefore formed by the moving lifting knives.
With this existing system the pulley device is a great disadvantage. At high weaving speed the reversing rollers of the pulley device have to rotate fast backward and forward. Heat develops through the friction which occurs in the bearing of the wheel and through slipping of the cord on the groove surface of the wheel. The cord must bendingly unwind onto the reversing roller at high frequency. This cord is subject to wear and tear and finally breaks. It also often occurs that through the dust in the weaving area the wheel will jam, through which the cord prematurely breaks through severe friction. After a time all pulley elements have to be preventively replaced when the number of pulley cord breakages becomes too great and because of this the weaving efficiency of the weaving installation will decrease. The replacement of thousands of pulleys per weaving unit is time-consuming, requires specialized personnel and because of this causes an increase in running costs.
EP 0 711 856 describes an attempt at remedying these disadvantages by operating without any pulley element. This device however has the disadvantage that a preselection of the hooks must take place with the implementation of a small lift at the frequency of the weft insertion frequency. In other words the selectors and the grids on which these are mounted must perform an upward and downward movement during a weft insertion cycle. This leads to severe vibrations at high operating speed of e.g. 1,000/min. Another disadvantage is that the lifting knives must be provided with spring catch hooks which drag against the jacquard hooks. This develops heat and is the cause of considerable mechanical loss.
Another attempt according to EP 0 779 384 also has the intention of being able to operate without pulley element. The disadvantage of that technique is that a two-legged hook is required whereby the harness load in each case comes in the middle, through which the hooks are eccentrically loaded. In order to offset this eccentric loading a central guiding body has to be provided. This however causes extra friction through which this device also suffers high mechanical losses. Because of the fact that this solution rests on a two-legged hook this device takes up rather a lot of room in horizontal plane. The footprint is rather large.
Shed-forming devices are also utilized in three-position jacquard machines such as namely those employed with face-to-face double gripper weaving machines for weaving jacquard velvet and for weaving multiple pile warp thread carpets. With a double gripper weaving machine in each weft insertion cycle two wefts are simultaneously inserted. This means that the pile warp threads can occupy three positions:
Bottom: below both weft insertion means PA0 Middle: between the two weft insertion means PA0 Top: above both weft insertion means.
It is called a three-position open-shed jacquard machine when each position of the three positions can be reached or continue to be maintained on every pick or weft insertion cycle. Three-position open-shed jacquard machines are implemented by providing two hooks of a two-position open-shed jacquard machine with a pulley device. The importance of three-position open-shed jacquard machines for weaving jacquard velvet and multiple pile warp thread carpets is that pile weave corrections can be applied at the time of color transitions where this appears necessary in order to avoid mixed contours and double tufts on the pile side when using the two-shot weave.
From the French patent publication no. 1.225.173 a three-position jacquard machine is known with open shed for the middle and bottom position and non-open shed for the top position. This device makes use of two card-operated hooks which are connected to each other by a pulley cord, which runs around the top wheel of a pulley device, and a bottom pulley cord which is secured to a movable grid and is re-routed over the bottom wheel in order then to be connected to the harness cord(s) with the other extremity. With this device the bottom and middle position can be reached or maintained on every pick, the top position can only be reached on every second pick. The disadvantage of this device is the use of pulley cords. Through the repeated passing around and the friction of the cords on the wheels, the cords are subject to wear and tear through which they will break. A device also has to be provided in order to move the bottom pulley grid.
From the French patent publication no. 1.513.410 a three-position open-shed jacquard machine is known which makes use of two hooks of a two-position open-shed jacquard machine and one pulley element. The device makes use of two hooks: this means that for a specific number of cords with three positions, a double capacity in hooks has to be installed. The pulley cords are here again the weak element of the device. With the higher weaving speeds, which are customary at present, the pulley cords break prematurely.
From the French patent publication no. 2 466 541 a similar device is known, but with a movement reinforcement built into the pulley device. The disadvantage of this device is also here the use of twice the number of hooks and pulley cords, and the extra reversing roller which is necessary for the movement reinforcement.
From the European patent publication no. 0 399 930 a device is known which makes use of two complementary hooks, each with its own pulley and one reversing roller in order to achieve the three-position open shed. With this pulley device the pulley cords are passed around in two planes standing perpendicular to each other through which the pulley cords break through fatigue and wear and tear of the fibers in the pulley cords. Here two neighboring hooks are also necessary in order to obtain a three-position device.
These known devices all have the disadvantage that the pulley cords of the pulley device are subject to wear and tear and that the pulley cords will break, which makes premature replacement necessary. This problem becomes more serious with current weaving speeds.
This invention now has the purpose of providing a shed-forming device which prevents the deficiencies and disadvantages of the state-of-the-art, and which is suitable for being used on jacquard devices of different types, namely two-position open-shed jacquard machines and three-position open-shed jacquard machines.